Impact of landscape composition and configuration on forest specialist and generalist bird species in the fragmented Lacandona rainforest, Mexico
Introduction
Continuous rainforests are being rapidly converted to human-modified tropical landscapes (HMTLs), largely due to accelerated human population growth and increasing demands for agricultural lands and forest products (Laurance et al., 2014). These landscapes are highly heterogeneous, showing different composition and spatial configuration, where composition refers to the types and proportions of different forms of land covers, and configuration refers to the spatial arrangement of a given land use/land cover class (sensu Dunning et al., 1992). Both landscape composition and configuration defines the heterogeneity or structure of a landscape, and can have different impacts on biodiversity (Fahrig, 2003, Fahrig et al., 2011, Tscharntke et al., 2012, Newbold et al., 2014). Nevertheless, the available information on this topic is very scarce as most studies in HMTLs have been performed at the patch scale, and do not assess, nor control for, the independent effects that landscape composition and configuration may have on biodiversity (Fahrig, 2003). Also, the majority of existing landscape-scale studies are focused on temperate ecosystems (McGarigal and Cushman, 2002). It is thus necessary to adopt a landscape perspective that distinguishes between composition and configuration in tropical ecosystems, to obtain a better understanding of species’ responses to land-use changes in HMTLs, and improve management and conservation strategies in the tropics (Gardner et al., 2009, Melo et al., 2013).
Studies from temperate forests evaluating the independent effects of landscape composition and configuration have usually found that landscape configuration (e.g. number or density of forest patches) shows a variable and weaker effect on biodiversity than landscape composition (e.g. habitat amount) (Trzcinski et al., 1999, Fahrig, 2003, Smith et al., 2011). In fact, fragmentation per se (i.e. the breaking apart of habitat after controlling for habitat loss; sensu Fahrig, 2003) can have negative, positive, or neutral effects on different taxa (Fahrig, 2003, Betts et al., 2006, Ethier and Fahrig, 2011). The negative effects can be related to the loss of landscape connectivity and the increase in forest edge (Murcia, 1995, Laurance et al., 2002). Nevertheless, by increasing the number of patches, fragmentation can promote the creation of subpopulations, favoring metapopulation dynamics and species persistence in more fragmented landscapes (Hanski, 1999). Other positive effects of fragmentation per se reviewed by Fahrig (2003) include: (1) increasing access to resources in forest edges, (2) reducing inter-patch isolation distances with increasing fragmentation per se; and (3) improving access from home patches to resources located in other neighboring patches, as indicated in the landscape complementation and supplementation hypotheses (Dunning et al., 1992).
In forested landscapes, variations in species responses to forest fragmentation and inter-patch isolation may be related to differences in the amount of old-growth forest cover in the landscape (Villard and Metzger, 2014) and to differences in matrix composition (Fahrig et al., 2011). Evidence indicates that the effect of forest fragmentation may be higher in landscapes with lower (Trzcinski et al., 1999) or intermediate forest cover (Villard and Metzger, 2014). At the same time, landscapes with heterogeneous matrices, such as those with live fences, isolated trees, secondary forests, and different types of land covers, can maintain more resources (Dunning et al., 1992, Fahrig et al., 2011) and higher landscape connectivity (Antongiovanni and Metzger, 2005, Fahrig, 2007), thus supporting more species than landscapes with homogeneous matrices (see the “landscape insurance hypothesis”; Tscharntke et al., 2012). For example, secondary forests are expected to contain resources for both forest specialist and habitat generalist birds (Blake and Loiselle, 2001), and also improve landscape connectivity for forest-specialist species (Stouffer and Bierregard, 1995, Pinotti et al., 2012). In contrast, open-habitat matrices can inhibit birds’ dispersal movements in HMTLs (Castellón and Sieving, 2006, Ibarra-Macias et al., 2011).
Changes in landscape composition and configuration not only alter patterns of local diversity (α), but also the composition and structure of remaining assemblages, potentially altering the species turnover (β-diversity) across multiple spatial scales (Flohre et al., 2011, Karp et al., 2012, Arroyo-Rodríguez et al., 2013, Püttker et al., 2015). For example, both increases (i.e. biotic differentiation) and declines (i.e. biotic homogenization) of β-diversity have been reported in HMTLs (Arroyo-Rodríguez et al., 2013). However, the relative impact of landscape composition and configuration on β-diversity remains poorly understood, particularly in the tropics (but see Karp et al., 2012).
Here we examined the relative effects of landscape composition and configuration on α- and β-diversity of birds within old-growth forest in a fragmented biodiversity hotspot – the Lacandona rainforest, Mexico. As landscape composition we considered the percentage of old-growth forest cover in the landscape and the percentage of secondary forest cover in the matrix. As measures of landscape configuration we included the number of old-growth forest patches and forest edge density. Since the responses of species to landscape composition and configuration can be scale-dependent (Smith et al., 2011, Thornton et al., 2011, Garmendia et al., 2013, San-José et al., 2014), these landscape metrics were measured at two spatial scales, within 100-ha and 500-ha landscapes. Also, as the impact of landscape change on species may vary according to the habitat specificity of species (Fahrig, 1998, Lindell et al., 2004, Pardini et al., 2010, Newbold et al., 2014), we separately assessed the response of forest interior specialist (those that use forest as the primary habitat) and habitat generalist species (those that use resources from different land covers and forest strata in HMTLs).
Based on the hypotheses and empirical evidence described above, we predicted that both α- and β-diversity will have stronger associations with landscape composition, than with landscape configuration, particularly when considering forest specialist species, which will be strongly and positively associated with the percentage of old-growth forest cover in the landscape. It has been reported that forest edge density in the landscape can have both positive and negative effects on biodiversity, depending on the vulnerability of the species to forest edge effects (reviewed by Ewers and Didham, 2006). Thus, we predicted that forest interior specialist species will have a negative association with forest edge density, whereas forest generalist species would be positively related to this factor because of increasing access to resources, in both forest edges and the neighboring matrix. Finally, regarding the composition of the landscape matrix, secondary forests are expected to contain more resources and refuges for both specialist and generalist birds than other covers of the matrix (Stouffer and Bierregard, 1995, Pardini et al., 2005, Banks-Leite et al., 2010); thus, we predicted that the diversity of birds will increase in patches surrounded by a matrix dominated by secondary forests.
This is a timely study for the conservation of birds, given that: (1) landscape-scale studies on tropical birds are very scarce (but see Cerezo et al., 2010, Kennedy et al., 2011); (2) the Lacandona region is one of the biologically richest Mexican ecosystems (González-García, 1993, Medellín, 1994); and (3) the region is experiencing a very high annual deforestation rate (annual rate of 2.1% between 1990 and 2010; Courtier et al., 2012), but to our knowledge, no study to date has evaluated the impact of landscape spatial changes on bird community in this vanishing biodiversity hotspot.
Section snippets
Study area
The Lacandona region is located in the southern part of Chiapas, Mexico (100–1500 m a.s.l.; Fig. 1), and represents the westernmost part of the Mayan forest, which extends through Guatemala to the Belize Mayan Mountains, and north to the middle portion of the Yucatan Peninsula. With an extent of 13,000 km2, this region represents one of the largest areas of tropical rainforest in Mexico, and a priority area for biodiversity conservation in Mesoamerica. Yet it has been strongly deforested during
Results
We recorded 84 bird species from 25 families (Table A2). Nine species (11%) were ubiquitous, occurring in all reference sites and in >80% of the patches. In contrast, 11 species (12%) were only recorded in reference sites, and 16 species (19%) were recorded exclusively in forest patches. Overall, 20 species (24% of all species recorded) were forest specialist, and 64 (71%) were generalist species (Table A2).
Discussion
Our results show that landscape composition and configuration affect the diversity of rainforest birds in the Lacandona region, Mexico, adding valuable information on the drivers of species diversity in HMTLs. Several of our findings can be highlighted. First, as predicted, and consistent with previous studies in temperate areas (e.g. Fahrig, 1998, Fahrig, 2003, Smith et al., 2011), old-growth forest cover showed the strongest association with the diversity of forest specialist species, with a
Acknowledgments
This research was funded by the Programa de Apoyo a Proyectos de Investigación e Innovación Tecnológica (PAPIIT), DGAPA-UNAM (Projects IA-203111, IB-200812 and RR-280812). EC obtained a scholarship from CONACyT, Mexico. The Centro de Investigaciones en Ecosistemas (UNAM) provided logical support. H. Ferreira and A. Valencia provided technical support, and A. Navarrete provided the SPOT images. C. Dobler digitized the maps. We are indebted to the local people of the Marqués de Comillas region.
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